Modeling of a Free Piston Energy Converter

Doctoral thesis, 2006

ABSTRACT The Free Piston Energy Converter (FPEC) is a new type of automotive prime mover, offering high efficiency in conjunction with low emissions. The FPEC essentially consists of two opposing cylinders with a linear alternator incorporated in-between that produces electric power directly. As indicated by the name, there is no mechanical linkage that controls the motion of the piston in the FPEC. Instead the piston moves as a response to the forces acting upon it, which implies that the piston motion will depend on operating conditions. Furthermore, it is possible to vary the compression ratio in simple manners, without any hardware modifications. However, by eliminating the crank mechanism a simple and effective way of control is removed, and control issues are the major drawback of the FPEC. Modeling was performed in order to investigate the multi-fuel capability of the FPEC, which is an inherent benefit of its variable compression. A dynamic model predicting the movement, velocity, acceleration and frequency of the translator was used in an iterative manner, together with the commercial 1-D gas dynamic code BOOST™, and detailed chemistry calculations in the SENKIN code. Reduced mechanisms of Diesel, gasoline, natural gas and hydrogen were used. Furthermore, detailed chemistry simulations employing a partially stirred reactor (PaSR) approach together with a detailed combustion mechanism (70 species participating in 306 reactions) of a Diesel oil surrogate, represented by a blend of n-heptane and toluene, were performed using the 3-D CFD code KIVA-3V. The effects of two consecutive cycles were modeled using a mesh representing the complete cylinder, whilst the effects of different injection schedules were studied using a sector mesh. Moreover, the effects of changing the scavenge port layout on the scavenging characteristics of a loop-scavenged engine were examined. Keywords: Free piston engine, combustion, variable compression, computational fluid dynamics (CFD), detailed chemistry